Silver allenes

When the chiral allenic amine 1 was cyclized using silver tetrafluoroborate, transfer of chirality occurred from the allenic moiety to the C-2 stereogenic center, as determined by conversion of 2 into the diastcrcomeric mixture of the (+)-(5)-0-methylmandelic amides of the debenzylated product. The high asymmetric induction was ascribed to the formation of a dissymmetric silver-allene complex229. 

Allene F Mild steel, aluminium, brass. Copper, silver and their alloys. 

IH of allenic amines also occurs in the presence of silver salts. IH of a-allenic amines proceeds in good yields in the presence of AgBp4 and provides a useful method for 3-pyrrolines synthesis via Endo-Trig processes (Eq. 4.93) [314]. 

Sediment Trap in silver nitrate. Gravimetry 0.01 pmol/g NR Allen et al. 1994... 

Clavepictines A and B (210 and 211, respectively) were obtained from the allenic ester 227. The reduction of its ester group to aldehyde, followed of addition to the latter of hexylmagnesium bromide, OH protection, and N-deprotection gave compound 228. A silver(i)-mediated cyclization of this compound afforded quinolizidine 229 and its C-6 epimer in a 7 1 ratio (Scheme 44). The former compound was readily converted into the target alkaloids <1997JOC4550>. 

Reaction (105) could also be induced in benzene at room temperature by addition of silver perchlorate. Of related interest is the decarboxylation of diketene either by an osmium cluster to give an i73-allyIosmium complex [Eq. (106)] (114) or by (T7-C5H5)Mn(CO)2(thf) to give (77-C5H5)-Mn(CO)2(allene) (115). 

Synthesis of a sugar derived allene and its intramolecular silver mediated etherification followed by ring closing metathesis has been explored for building the tricyclic framework of eunicin (Fig. 52).68... 

The [3,3]-sigmatropic rearrangement ofpropargyl esters [234, 561] is usually an equilibrium reaction for example steric repulsion can help to deliver mainly the allene [562] and faster reaction kinetics are observed with silver(I) and copper(I) catalysts [562-571] (see cyclopropane 169 in Scheme 1.75) [572], and recently also rhodium(I) catalysts [573]. 

A further example of the reductive allene formation in the synthesis of a non-alle-nic natural product was reported recently by VanBrunt and Standaert (Scheme 2.47) [81]. Treatment of the propargylic silyl ether 147 with LiAlH4 led to the syn-stereose-lective formation of the hydroxyallene 148, albeit with unsatisfactory chemical yield (25-50%). The latter was then transformed into the antibiotic amino acid furanomy-cin (150) by silver-mediated cycloisomerization to dihydrofuran 149 and elaboration of the side-chain. 

The stereospecific reduction of a 2-butyne-l, 4-diol derivative and silver( I)-mediated cyclization of the resulting allene were successively applied to a short total synthesis of (+)-furanomycin 165 (Scheme 4.42) [68], Stereoselective addition of lithium acetylide 161 to Garner s aldehyde in the presence of zinc bromide afforded 162 in 77% yield. The hydroxyl group-directed reduction of 162 with LiAlH4 in Et20 produced the allene 163 stereospecifically. Cyclization followed by subsequent functional group manipulations afforded (+)-furanomycin 165. 

The cydization can also be carried out on a-tosylaminoallenes, in which case the choice of reaction conditions determines whether the product is the N-tosyl-3-pyrro-line or whether elimination of toluenesulfonic acid acid gives the pyrrole. For example, in the presence of catalytic silver nitrate, allene 141 (Eq. 13.47) rearranges to N-tosylpyrroline 142 in excellent yield, whereas when 141 is treated with potassium tert-butoxide in DMSO, pyrrole 143 is formed in 71% yield [54]. Warming the lithium salt of 141 in DMSO also leads to 143. The rearrangement of 141 to 143 may be mechanistically related to the conversion of 130 to 131 (Eq. 13.42). 

Application of silver-catalyzed cydization is a key step in the synthesis of clavepic-tines A and B, a synthesis which also established the absolute configuration for these compounds. With regard to the allene unit and the heterocycle, enantiomeri-cally pure precursor 129 was prepared and then cyclized to the quinolizidine 130 with AgN03 in a diastereoselective manner (Scheme 15.40) [89, 90]. The synthesis was conducted with an inseparable 1 1 mixture of diastereomers at C-14 from the diastereomerically pure allene with regard to the axial chirality of the allene a 7 1 mixture of diastereomers (at C-10) was formed. 

The 2-vinylpyrrolidines 321 are formed in up to 99% diastereomeric excess by the ring closure of the allenic sulphides or sulphoxides 320 (R = PhS or PhSO), induced by silver(I) ions376. 

Ultrasound irradiation of mixtures of amines RNH2 (R = PI1CH2, Ph or Ar) and methyl pyruvate results in the 3-pyrrolin-2-ones 322377. The silver tetrafluoroborate-catalysed cyclization of the allenic amines 323 leads either to a pyrroline 324 or tetrahydropyridine 325, depending on the structure of the amine. The former is formed from 323 (R = H), the latter from 323 (R = Me)378. 

FIG. 3. Geometry of hydrated molecules cylinders associate to a lamellar liquid crystal, cones to a hexagonal and an inverse hexagonal. Adapted from The Physical Chemistry ofMembranes (Silver, B., ed.), Allen Unwin, Inc. Solomon Press, Winchester, MA, 1985. 

Silver, B, ed.. The Physical Chemistry of Membranes, Allen Unwin, Ine., Solomon Press, Winchester, MA, 1985. 

Addition of the lithiated allene to an imine yielded a mixture of four diastereoisomers (ratio 68/17/12/3 68% ds) which was subjected to a silver nitrate induced cyclization. Product 145 derived from major diastereoisomer 144 was isolated and further converted to (-)-preussin (Scheme 36). 

Various types of diethylphosphono-substituted a-allenic alcohols have been transformed to the corresponding 4-(diethylphosphono)-2,S-dihydrofurans by treatment with a catalytic amount of silver(I) nitrate under a nitrogen atmosphere <99S463>. 

At the end of 2007, Widenhoefer et al. reported the first examples of the dynamic kinetic enantioselective hydroamination of axially chiral allenes, catalyzed by a dinuclear complex of gold (Figure 8.1) and silver perchlorate [46, 47]. 

Silver(I) catalyzed cyclizations of allenic alcohols (202) lead to 2,5-dihydrofurans (203) (79S743), whilst another mild method for the synthesis of tetrahydrofurans is the intramolecular oxymercuration-demercuration process. Geraniol, when treated with mer-cury(II) acetate and subsequently with sodium borohydride, gave a tetrahydrofuran. 

The conversion of /3-allenic alcohols into 5,6-dihydro-2F/-pyrans is catalyzed by silver ions and takes place under mild conditions (79S743). 

When treated with catalytic amounts of acid,404 403 silver salts406"410 or mercury salts,409,410 allenic alcohols will cyclize to ethers (equations 230-253). By proper choice of reaction conditions, the yields of products obtained in these cyclizations are generally high. Interestingly, the stereoselectivity observed in equation (253) is proposed to arise from a chair conformation of the intermediate.408... 

Allenes can also act as the -participant in electrophilic heteroatom cyclizations. Reviews of electrophilic additions to allenes discuss early examples of this type of cyclization.ld le-202 Numerous examples of cyclizations of a-functionalized allenes, including carboxylic acids, phosphonates, sulfinates and alcohols, to form five-membered heterocycles (equation 84) are cited in these reviews. The silver nitrate-mediated conversion of ot-allenic alcohols to 2,5-dihydrofurans203 has recently been applied to trimethylsilyl-substituted systems.204... 

Cyclization of a variety of y-allenic alcohols with silver nitrate proceeds by 5-exo cyclization to form 2-alkenyltetrahydrofurans (equation 87).205c 206 Little stereoselectivity is seen in cyclizations of secondary alcohols. Cyclization by intramolecular oxypalladation/methoxycarbonylation or oxymercuration followed by transmetallation and methoxycarbonylation also showed no stereoselectivity (equation 88 and Table 24, entries 1 and 2).50 207 However, cyclization of the corresponding r-butyldimethylsilyl ether derivatives with mercury(II) trifluoroacetate followed by transmetallation/methoxycarbonylation pro-... 

Allenic alcohols with a variety of allene substitution patterns form 2-alkenyltetrahydropyrans by a 6-exo ring closure upon cyclization with silver or mercury salts (equation 91 ).209 The organosilver intermediate undergoes protiodemetallation under the reaction conditions, while the vinylmercury... 

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